1. Field of the Invention
The present invention relates to a process for industrially producing methyl methacrylate by the use of methyl .alpha.-hydroxyisobutyrate as the starting raw material. Methyl methacrylate has industrially significant uses, for example, as the starting raw material for poly(methyl methacrylate) which is excellent in weather resistance and transparency, those for a variety of methacrylic acid esters, and the like.
2. Description of the Related Arts
The present inventors provided in Japanese Patent Application Laid-Open No. 196,753/1990, a process for producing an .alpha.-or .beta.-unsaturated carboxylic acid ester by subjecting an .alpha.-hydroxycarboxylic acid ester or an .alpha.-or .beta.-alkoxycarboxylic acid ester alone or the mixture thereof as starting raw materials to dehydration or dealcoholization reaction by the use of a crystalline aluminosilicate as the catalyst. Among the crystalline aluminosilicates used in the above-mentioned process, type-X or type-Y zeolite exhibited particularly excellent catalytic-activity. There are also disclosed that the crystalline aluminosilicate modified with an alkali metal and/or a platinum group element, especially type-X or type-Y zeolite is particularly effective as the catalyst in Japanese Patent Application Laid-Open No. 167,155/1991, 167,156/1991 and 167,157/1991.
As a result of further investigation based on the above-mentioned information, it has been found that such problems have been raised by the use of as the catalyst, ordinary type-X or type-Y zeolite or type-X or type-Y zeolite modified with an alkali metal and/or a platinum group element that the catalyst considerably deteriorates in a short period depending on the reaction conditions, thus requiring frequent regeneration thereof and besides the coloring phenomenon of the reaction liquid makes it difficult to separate the coloring substance from the objective product, thereby increasing the burden on the product purification step.
The early deterioration of the catalyst which participates in the reaction makes it impossible to continue the reaction after several days to a few months from the start of the reaction depending upon the reaction conditions. The deteriorated catalyst can be regenerated by firing it at a temperature higher than the reaction temperature, but frequent regeneration procedure is not favorable from the standpoint of industrial stabilized operation. In addition, the coloring substance appearing in the reaction liquid has a boiling point close to that of methyl methacrylate and a strong affinity therefor, and hence the substance can not easily be removed by separation through the procedure of distillation, extraction or the like. On the other hand, further improvement in the quality of methyl methacrylate as the product has increasingly been desired with the recent trend directed to the higher quality and sophistication not only in the field of special purpose including optical fiber but also in the field of general purpose such as molding material, extrusion plates, casting plates and coating materials. Accordingly, it is strongly required to suppress during the course of reaction the formation of the coloring substance.